This invention relates to a multipath detection and avoidance method and system, and more particularly to such a method and system for identifying multipath errors from a moving radiation source such as a GPS satellite and eliminating their effect on the performance of the receiver.
The global positioning system (GPS) employs a GPS receiver responsive to the signals from a number of GPS satellites to determine the position of the receiver which may be borne by a person, airplane, missile, boat, car or other object. In more advanced applications the GPS may be used to determine not just the position but the attitude (e.g., the spatial orientation) of an object. For example, satellite carrier phase measurements from a plurality of antennas placed on the body, wings and tail, of an airplane can be processed by interferometric phase comparison to determine the roll, pitch and yaw of an airplane. However, attitude determination techniques have not achieved the accuracy predicted based solely on measurement noise considerations. The widely held belief is that multipath propagation is the cause of the shortfall. Normally in interferometric based attitude determination the signals from a satellite are sensed by two spaced antennas. The phase difference in the wavefronts arriving simultaneously at one and the other of the antennas encodes the angle between the antenna baseline and the line of sight from the antennas to the satellite. By applying this analysis to signals received from a number of satellites and combining them the position and attitude of the receiver and the antenna baseline may be determined. However, multipath propagation occurs when the signal from a satellite arrives at the antennas not just along a single direct path but also through one or more reflective paths so that multiple signals are received by the antennas thereby introducing errors in the position and attitude determinations.
It is therefore an object of this invention to provide a multipath detection method and a system to suppress the errors that this multipath can cause in the operation of the receiving system.
It is a further object of this invention to provide such a multipath detection method and suppression system for a global positioning satellite system receiver as well as for any communication navigation system that employs carrier phase modulation.
It is a further object of this invention to provide such a multipath detection method and system which identifies multipath error and corrects or removes it.
It is a further object of this invention to provide such a multipath detection method and system which can be used to map the position and attitude of reflecting surfaces that cause multipath.
The invention results from the realization that multipath distortion of signals from moving radiation sources such as GPS satellites can be detected by comparing the time history of the phase difference of a carrier signal from a moving source in a particular location as sensed by a pair of carrier phase interferometric antennas with the ideal or predetermined phase difference for a source in that location and analyzing any difference in those phase differences to detect the presence of a multipath error signal which can be eliminated or corrected or used to map the position of the reflector causing the multipath error.
This invention features a multipath error detection system including first and second spaced antennas for receiving a carrier signal from a moving radiation source and producing a carrier phase difference representative of the location of the source. There are means responsive to the carrier phase difference and to the predetermined phase difference for the source in that location for generating a difference signal representative of any difference in those phase differences. There are also means responsive to the difference signal for determining the presence of a multipath error signal. The technique can be extended in applicability and effectiveness to implementations in which there are more than one pair of antennas.
In a preferred embodiment there are means for removing the multipath error signal. The means for removing multipath may include means for eliminating the carrier signal containing the multipath error signal or may include means for correcting the multipath error signal in the carrier signal. The system may further include means for identifying the position of the reflector which generates the multipath error signal.
The invention also features a multipath error detection method including receiving from first and second spaced antennas a carrier signal from a moving radiation source and producing a carrier phase difference representative of the location of the source. There is generated a difference signal representative of any difference in the phase differences between the measured carrier phase difference and a predetermined phase difference for the source in that location. The presence of a multipath error signal in the difference signal is then determined.
In a preferred embodiment the method may further include removing the multipath error signal such as by eliminating it from the carrier signal or correcting the multipath error signal and the carrier signal. The method may further include identifying the position of the reflector which generates the multipath error signal.